The application of cementitious compositions such as concrete to a substrate by spraying from a nozzle is a well-established technology, and is widely used in such applications as the lining of tunnels. It is a requirement that such compositions are able to be easily conveyed (usually by pumping) to a spray nozzle. This can be achieved by the addition to the cementitious composition that is to be pumped and sprayed, at the mix stage, of an admixture which confers improved fluidity of the mix. There is a considerable variety of such admixtures known to and used by the art, for example, sulfonate melamine formaldehyde condensate, sulfonate naphthalene formaldehyde condensate, or acrylic polymer families.
Sprayed concrete or “shotcrete” is mainly used in underground construction. Its application consists in the conveying of a mortar or concrete to a nozzle, where a set accelerating admixture and air are added, and it's pneumatically projection at high velocity onto a substrate. Indeed, fast setting and early strength development are needed to allow concrete adhesion on the wall without falls and hence earlier entrance and further excavation are ensured, guarantying security and efficient construction. For this reason, accelerators which ensure rapid development of the mechanical properties are added to the sprayed concrete or sprayed mortar.
It is known that set accelerators influence the hydration process of clinker phases such as C3A and C3S, the consumption of the sulfate carriers and the chemical composition of pore solution at the very beginning stage. The quicker setting caused by adding set accelerators is due to large formation of hydration product like ettringite, resulting in some cases with a poor early strength development which is in the literature correlated to a slow calcium silicate hydrates (C—S—H) formation.
By adding (1) a hardening accelerator consisting in calcium silicate hydrates and (2) a set accelerator the calcium silicate hydrate (C—S—H) precipitation can be strongly accelerated allowing faster development of the mechanical properties.
A process for the preparation of a liquid accelerator is described in EP 08170692.1, wherein the liquid accelerator containing aluminum sulfate and/or aluminum hydroxy sulfate. EP1878713 describes an accelerating admixture that is based on 25 to 40% by weight of aluminum sulfate, at least one further aluminum compound, so that the molar ratio of aluminum to sulfate in the dispersion is 1.35 to 0.70 and an inorganic stabilizer, which comprises a magnesium silicate. A process of applying a layer of cementitious composition on a substrate by spraying the cementitious composition is described in EP 0812812. An accelerating admixture and hardening accelerator for hydraulic binder according to EP 1964825 comprises sulfate, aluminum, organic acid and/or mineral acid and silicic acid. WO 2005/075381 describes a water-based accelerating admixture and a hardening accelerator for hydraulic binder, comprising sulfate, aluminum and organic acid, wherein the molar ratio of aluminum to organic acid is less than 0.65.
Conventional setting accelerators for shotcrete and other cement-containing materials are distinguished in that they either result in rapid setting and relatively low early strength or lead to slow setting in combination with relatively high early strength. Furthermore, the results with respect to the set accelerator performance in concrete vary often depending on the cement type, concrete mix design, and temperature. Even though all these factors are taken into account before application, it is difficult to achieve the high early strength development that nowadays are required for an increasing number of tunneling projects with known set accelerators. Therefore, the demand for more efficient and highly robust accelerator admixtures, which are suitable for a wide variety of cements on a world-wide basis and comply with miscellaneous specification, is high.
Furthermore, admixtures for building material mixtures comprising hydraulic binders typically also contain hardening accelerators which increase the early strength development rate of the hydraulic binder.
According to WO 02/070425, calcium silicate hydrates, can be used as such a hardening accelerator. However, commercially available calcium silicate hydrates and dispersions thereof may be regarded only as hardening accelerators which have little effect.
The object of the invention is to provide a sprayable hydraulic binder composition as e.g. sprayed concrete or sprayed mortar for the coating of substrates, in particular tunnel surfaces, mine surfaces, construction trenches and shafts, with concrete or mortar.
Surprisingly it has been found a new chemical system to accelerate setting and early strength development in hydraulic binder compositions, in particular in sprayed concrete or sprayed mortar.